1

CO2 Gas Absorption Experiment

By: Stephen LaBonia

2

Table of Contents Abstract ........................................................................................................................................... 1

Introduction ..................................................................................................................................... 1

Objectives ........................................................................................................................................ 1

Experimental Procedure .................................................................................................................. 2

Materials ..................................................................................................................................... 2

Equipment ................................................................................................................................... 2

Experimental Design ................................................................................................................... 6

Experimental Protocol ................................................................................................................ 8

Safety .......................................................................................................................................... 8

Results and Discussion..................................................................................................................... 9

Conclusions .................................................................................................................................... 11

Acknowledgements ....................................................................................................................... 12

References ..................................................................................................................................... 12

Abstract Through UF's IPPD program, our team constructed a carbon dioxide gas absorption system in the Unit

Operations lab with the intent for it being used as a future experiment for the Unit Operations course.

Standard operating procedures and system overview documents have already been written so future

students can operate the experiment as well as complete experimental objectives that were created.

Data has already been collected and analyzed to complete these objectives and serve as an example of

data that future students could collect. The chemicals used in the experiment include carbon dioxide

and anhydrous potassium carbonate that is used to make a 30 wt/wt% solution. A MSDS is also being

provided as well as a form for students to properly record data during the experiment.

Introduction CO2 gas emissions are becoming a greater impact on the environment, and new technology regarding

absorbing this pollutant is being researched. One of these technologies is using a gas absorption

column. Solvent is introduced counter-currently to an incoming gas stream where chemical and

physical absorption occur in a packing material in the center of the column. This material slows the

flowrate of both the gas and liquid and allows for greater absorption. The Department of Energy is

currently funding different methods to improve the efficiency of CO2 absorption at numerous

universities across the nation. This report goes over absorption tower technology and the different

parameters that can be changed to optimize CO2 absorption.

Objectives Compare experimental L/V ratios with ratios found through using McCabe-Thiele analysis for

low concentration system with random packing material

Compare CO2 absorbance with changing L/V ratios

Compare absorption of CO2 using water and solvent

Calculate mass transfer coefficient for range of L/V ratios

2

Experimental Procedure

Materials Table 1: Materials used during experiment

Materials Usage Properties

Water Wash out absorption column

and collect absorption data

-Physical state: Liquid

-Molecular Weight: 18.02

g/mol

-Density: 0.9970474 g/mL at

25°C

-Viscosity: 8.9x 10-4 Pa.s

Potassium Carbonate

(Anhydrous)

Create a 30 wt/wt % solution to

act as solvent

-Physical state: solid

-Molecular Weight: 138.21

g/mol

-Density: 2.43 g/mL at 19°C

-Solubility: Soluble in water

-pH: 11.6 (10% aq solution)

Carbon Dioxide Chemical that will be absorbed

from the gas phase

-Physical state: gas

-Molecular Weight: 44 g/mol

-Density: 1.98 kg/m^3 at STP

Equipment The system uses a combination of sensors, pumps, and tubing to transport both gas and solvent

throughout the experiment. Figure 1 below shows the process flow diagram for the experiment as

well as labels for all valves and equipment.

3

Figure 1: Process Flow Diagram for CO2 Gas Absorption System

The experimental system spans all three floors of the unit operations lab and each section below

shows what equipment is found on each of these floors:

First Floor:

Figure 2 lays out the connections to the first story solvent storage tank. This is where solvent will be

measured and added to the system and pumped up to the third story storage tank. This first story tank will

4

also work as a holding tank for solvent to be recirculated through the system again after passing through

the column.

Figure 2: First floor configuration of CO2 gas absorption system

Second Floor:

Figure 3 shows the control panel for the experiment with each flowmeter labeled so the user can know

which flow they are controlling. Two sensors are also placed on the board and monitor pH of the solvent

as well as the CO2 concentrations of inlet and outlet gas streams to the column.

Transfer Pump

1st

Floor

Storage Tank

Pump Air

Supply

5

Figure 3: Control Panel

Third Floor:

Figure 4 displays the third story solvent storage tank. This is where solvent will be mixed through the use

of a mixing pump before being sent to the flowmeter located on the control board.

Figure 4: Third floor configuration of CO2 gas absorption system

6

Experimental Design The objective of this experiment is to alter the liquid/gas ratio in the absorption column and notice

how it plays a role on the absorption of CO2. The flowrate of gas will remain constant while only

solvent flowrate is changed. Data will be organized using the following format for each trial ran:

Table 2: Format for data collection

Time Co2 % in CO2 % out pH

Before data can be accurately collected, calibrations must also be done for the CO2 and solvent

flowmeters as they are not calibrated from the manufacturer for these chemicals. Calibrations for CO2

flow rate were done using Equation 1 below and gave the following calibration curve shown in Figure

5.

𝐹𝐶𝑂2 = 𝐹𝐴𝑖𝑟

𝑆𝐺𝐴𝑖𝑟𝑆𝐺𝐶𝑂2

0.5

1

7

Figure 5: CO2 rotameter calibration

Calibrations for the solvent flowmeter were done by opening valve SV-4 and recording the time it

took for the flow to fill up a graduated cylinder. This data was collected for a range of flowrates and

gave the following calibration curve shown in Figure 6:

Figure 6: Solvent rotameter calibration

The reason for the great difference between measured and actual flowrate is due to the difference in

density between water and 30 wt% K2CO3; potassium carbonate is 40% more dense.

8

Experimental Protocol Vary Liquid/Gas ratio in the column:

Ensure solvent is thoroughly mixed and configure valves to flow to second story control board

Open compressed air and CO2 lines and configure their respective rotameters to achieve an

inlet CO2 concentration of 13.5%

Open valves and solvent flowmeter to flow solvent to column and down to the first story

storage tank.

Monitor pH and outlet concentration of CO2

Allow system to reach steady state before changing to a different solvent flowrate

Safety a) Personal Protective Equipment

a. Hard hat

b. Safety glasses

c. Long pants

d. Closed-toe shoes

e. N-95 Respirator (if handling anhydrous chemical)

f. Nitrile gloves (if handling solvent)

b) Safety Hazards and Prevention

Table 3: Safety hazards and prevention

Safety Hazard Prevention

Trip/Slip -Remove trip objects

-Locate tubes and valves near floor

Chemical Spills

- Follow instructions outlined at

http://ww2.che.ufl.edu/unit-ops-

lab//safety.htm

Potassium Carbonate (Anhydrous)

(Harmful lung irritant)

- Sign and review safety procedures for

wearing N-95 respirator mask

- Wear mask whenever in direct contact

with solid

- Do not wear contact lenses as the

solid particles can get into eyes

9

Potassium Carbonate (30 wt/wt % solution) - Wear nitrile gloves when handling

solvent due to its corrosive properties

CO2 Heater

- Heater should not be touched while in

operation but if needed, wear steam

rated heat gloves when handling

Results and Discussion During experimental runs, our team was able to observe CO2 absorption as well as trends for when

Liquid/Gas ratio changes. The data collected was all compiled into one graph (Figure 7) that compares

time and % CO2 absorbed through the system using both water and K2CO3:

Figure 7: CO2 Absorption data vs time

As can be seen from the graph, CO2 absorption decreases as liquid/gas ratio also deceases. This was

an expected trend because if you lower the solvent flowrate, the reaction between the CO2 and solvent

will decrease. The graph includes linear trend lines as a way to show this decrease. The reason for the

jumps between different flowrates is due to our team changing the flowrates while running the

0

5

10

15

20

25

30

35

40

0 10 20 30 40 50 60 70

% C

O2

Ab

sorb

ed

Time (min)

CO2 Absorbance

L/V = 12.5 [L/m^3]

L/V = 10 [L/m^3]

L/V = 7.5 [L/m^3]

Water @ L/V = 12.62 [L/m^3]

Linear (L/V = 12.5 [L/m^3])

Linear (L/V = 10 [L/m^3])

Linear (L/V = 7.5 [L/m^3])

Linear (Water @ L/V = 12.62[L/m^3])

10

experiment and waiting for steady state to be reached again. Once steady state was reached for each

experimental run, the solvent flowrate was decreased. Water was also plotted alongside potassium

carbonate and showed another trend that was predicted. Water absorbed a significantly less amount of

CO2 than K2CO3. Water has a much higher Henrys constant than potassium carbonate which allows

for only a small amount of CO2 to actually be absorbed.

One unexpected phenomena occurred however, which was the absorbance of CO2 without any solvent

running through the column. Without solvent flow, the outlet CO2 concentration should read 13.5%

but would always read on average around 9.7%. We are unsure how this absorption could occur unless

there is some absorbance between the CO2 and the wet packing material. To account for this, all

percent absorbance shown in Figure 7 are calculated assuming the inlet concentration is the outlet

concentration with no solvent flow running.

The experimental data collected was then compared to predicted values from using a McCabe-Thiele

analysis assuming gas and liquid flows have low concentrations of CO2. The equilibrium line for the

McCabe-Thiele analysis was found using the following VLE data for K2CO3 and CO2. A tangent line

was drawn at the center of the curve so we could assume the Henrys constant would remain constant.

This VLE data is shown below in Figure 8 and gave our team a Henrys constant value of 6.13.

Figure 8: VLE data for 30 wt% potassium carbonate solution at 40 Celsius [1]

An operating line was then formulated assuming a CO2 input of 13.5% and an output concentration

equal to what was experimentally measured. Figure 9 below shows the McCabe-Thiele analysis for

only one of three Liquid/Gas ratios tested.

11

Figure 9: McCabe-Thiele Analysis for L/G ratio of 3

To reach the desired CO2 outlet concentration from what was experienced during the experiment, the

McCabe-Thiele analysis suggests a molar flowrate Liquid/gas ratio of 3. The experimental run,

however, was ran at a molar flowrate Liquid/Gas ratio of 12 (which corresponds to L/V = 12.5

[L/m^3]). The difference in the expected vs theoretical ratios may be due to the Henrys constant

chosen. In the future, this constant could be found through benchtop experimentation in the Unit

Operations lab and could show better predictions.

Regarding the calculations for the overall mass transfer coefficient, its value did not change for each

of the three liquid/gas ratios tested. This could be due to how the number of equilibrium stages was

found. They were approximated using drawing tools on the graph where they could accurately be

calculated through the use of coding. The value for the overall mass coefficient found was 22,700

mol/(m^3*h).

Conclusions Overall, the gas absorption system showed expected trends regarding the effect of CO2 absorption

with respect to Liquid/Gas ratio. As the Liquid/Gas ratio was decreased, the percent CO2 absorbed

decreased as well. At peak absorbance, the system was able to absorb approximately 29%. This

absorbance takes into account the background absorption caused by the wet packing material with no

solvent flow. Experimental data was then compared to theoretical predictions using the McCabe-

12

Thiele analysis for low concentration systems. There was a significant difference between the two and

this could be caused by an inaccurate Henrys constant for potassium carbonate 30 wt% solution. An

overall mass transfer coefficient was also calculated and was the same for all Liquid/Gas ratios tested

which should not be the case. This could be caused by how the number of theoretical stages was

calculated. If more precise methods could be used to find the number of stages, a more accurate

coefficient could be calculated.

Acknowledgements I would like to acknowledge Dr. Rivera and the rest of my IPPD team for the help in constructing and

designing this experiment.

References 1. https://www.sciencedirect.com/science/article/pii/S0378381217302200